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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="the_type_traits_introspection_library.tti_detail_has_template"></a><a class="link" href="tti_detail_has_template.html" title="Introspecting an inner class template">Introspecting
    an inner class template</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="tti_detail_has_template.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro">Using
      the BOOST_TTI_HAS_TEMPLATE macro</a></span></dt>
<dt><span class="section"><a href="tti_detail_has_template/tti_detail_has_template_metafunction.html">Using
      the has_template_(xxx) metafunction</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro"></a><a class="link" href="tti_detail_has_template.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro" title="Using the BOOST_TTI_HAS_TEMPLATE macro">Using
      the BOOST_TTI_HAS_TEMPLATE macro</a>
</h3></div></div></div>
<p>
        The TTI macro <code class="computeroutput"><a class="link" href="../BOOST_TTI_HAS_TEMPLATE.html" title="Macro BOOST_TTI_HAS_TEMPLATE">BOOST_TTI_HAS_TEMPLATE</a></code>
        introspects an inner class template of a class. The macro must specify, at
        the least, the name of the class template to introspect.
      </p>
<h5>
<a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.h0"></a>
        <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.two_forms_of_introspection"></a></span><a class="link" href="tti_detail_has_template.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.two_forms_of_introspection">Two
        forms of introspection</a>
      </h5>
<p>
        There are two general forms of template introspection which can be used.
        The first is to find a class template with any number of only template type
        parameters ( template parameters starting with <code class="computeroutput"><span class="keyword">class</span></code>
        or <code class="computeroutput"><span class="keyword">typename</span></code> ). In this form
        only the name of the class template needs to be specified when invoking the
        macro. We will call this form of the macro the <code class="computeroutput"><span class="keyword">template</span>
        <span class="identifier">type</span> <span class="identifier">parameters</span></code>
        form. An example of a class template of this form which could be successfully
        introspected would be:
      </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">X</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">Y</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">Z</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">AClassTemplate</span> <span class="special">{</span> <span class="comment">/* etc. */</span> <span class="special">};</span>
</pre>
<p>
        The second is to find a class template with specific template parameters.
        In this form both the name of the class template and the template parameters
        are passed to the macro.
      </p>
<p>
        We will call this form of the macro the <code class="computeroutput"><span class="identifier">specific</span>
        <span class="identifier">parameters</span></code> form. An example of
        a class template of this form which could be successfully introspected would
        be:
      </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">X</span><span class="special">,</span> <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">Y</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">Z</span><span class="special">&gt;</span> <span class="identifier">BClassTemplate</span> <span class="special">{</span> <span class="comment">/* etc. */</span> <span class="special">};</span>
</pre>
<p>
        When using the specific form of the macro, there are two things which need
        to be understood when passing the template parameters to the macro. First,
        the type of the template parameters is relevant but the actual names of the
        template parameters passed are irrelevant. The actual names can be left out
        completely or be different from the names in the nested class template itself.
        Second, the use of 'typename' or 'class', when referring to the type of a
        template parameter, is completely interchangeable, as it is in the actual
        class template itself.
      </p>
<h5>
<a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.h1"></a>
        <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.variadic_and_non_variadic_macro_usage"></a></span><a class="link" href="tti_detail_has_template.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.variadic_and_non_variadic_macro_usage">Variadic
        and non-variadic macro usage</a>
      </h5>
<p>
        When using the BOOST_TTI_HAS_TEMPLATE macro we distinguish between compilers
        supporting variadic macros or not supporting variadic macros.
      </p>
<p>
        The programmer can always tell whether or not the compiler supports variadic
        macros by checking the value of the macro BOOST_PP_VARIADIC after including
        the necessary header file <code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">tti</span><span class="special">/</span><span class="identifier">has_template</span><span class="special">.</span><span class="identifier">hpp</span></code> in order to use the BOOST_TTI_HAS_TEMPLATE
        macro. A value of 1 indicates the compiler supports variadic macros while
        a value of 0 indicates the compiler does not support variadic macros.
      </p>
<p>
        Modern C++ compilers, in supporting the latest C++11 standard, normally support
        variadic macros. Even before the latest C++11 standard a number of C++ compilers
        already supported variadic macros. If you feel your compiler supports variadic
        macros and BOOST_PP_VARIADIC is 0 even after including <code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">tti</span><span class="special">/</span><span class="identifier">has_template</span><span class="special">.</span><span class="identifier">hpp</span></code>, you can predefine BOOST_PP_VARIADIC
        to 1 before including <code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">tti</span><span class="special">/</span><span class="identifier">has_template</span><span class="special">.</span><span class="identifier">hpp</span></code>.
      </p>
<h5>
<a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.h2"></a>
        <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.non_variadic_macro_usage"></a></span><a class="link" href="tti_detail_has_template.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.non_variadic_macro_usage">Non-variadic
        macro usage</a>
      </h5>
<p>
        We start with syntax for compilers not supporting variadic macros since this
        syntax can also be used by compilers which do support variadic macros. The
        form for non-variadic macros always takes two macro parameters. The first
        macro parameter is always the name of the class template you are trying to
        introspect.
      </p>
<p>
        The second macro parameter, when using the <code class="computeroutput"><span class="identifier">specific</span>
        <span class="identifier">parameters</span></code> form of the macro, is
        the template parameters in the form of a Boost preprocessor library array
        data type. When using the <code class="computeroutput"><span class="keyword">template</span>
        <span class="identifier">type</span> <span class="identifier">parameters</span></code>
        form of the macro the second macro parameter is BOOST_PP_NIL. If the second
        parameter is neither a Boost preprocessor library array data type or BOOST_PP_NIL
        you will get a compiler error if your compiler only supports non-variadic
        macros.
      </p>
<p>
        The non-variadic macro form for introspecting the class templates above using
        the <code class="computeroutput"><span class="keyword">template</span> <span class="identifier">type</span>
        <span class="identifier">parameters</span></code> form would be:
      </p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">AClassTemplate</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span>
<span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">BClassTemplate</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span>
</pre>
<p>
        Invoking the metafunction in the second case would always fail since the
        BClassTemplate does not have all template type parameters.
      </p>
<p>
        The non-variadic macro form for introspecting the class templates above using
        the <code class="computeroutput"><span class="identifier">specific</span> <span class="identifier">parameters</span></code>
        form would be:
      </p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">AClassTemplate</span><span class="special">,(</span><span class="number">4</span><span class="special">,(</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">typename</span><span class="special">,</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">typename</span><span class="special">)))</span>
<span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">BClassTemplate</span><span class="special">,(</span><span class="number">3</span><span class="special">,(</span><span class="keyword">class</span><span class="special">,</span> <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">class</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)))</span>
</pre>
<p>
        You need to be careful using the non-variadic <code class="computeroutput"><span class="identifier">specific</span>
        <span class="identifier">parameters</span></code> form to specify the
        correct number of array parameters. This can sometimes be tricky if you have
        a template template parameter, or a non-type template parameter which has
        parentheses surrounding part of the type specification. In the latter case,
        when parentheses surround a comma ( ',' ), do not count that as creating
        another Boost PP array token. Two examples:
      </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">void</span> <span class="special">(*</span><span class="identifier">FunctionPointer</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">long</span><span class="special">)&gt;</span> <span class="keyword">class</span> <span class="identifier">CClassTemplate</span> <span class="special">{</span> <span class="comment">/* etc. */</span> <span class="special">};</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">DClassTemplate</span> <span class="special">{</span> <span class="comment">/* etc. */</span> <span class="special">};</span>

<span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">CClassTemplate</span><span class="special">,(</span><span class="number">1</span><span class="special">,(</span><span class="keyword">void</span> <span class="special">(*)(</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">long</span><span class="special">))))</span>
<span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">DClassTemplate</span><span class="special">,(</span><span class="number">2</span><span class="special">,(</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">class</span><span class="special">)))</span>
</pre>
<p>
        In the case of using the macro to introspect CClassTemplate the number of
        Boost PP array parameters is 1, even though there is a comma separating the
        tokens in <code class="computeroutput"><span class="keyword">void</span> <span class="special">(*</span><span class="identifier">FunctionPointer</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">long</span><span class="special">)</span></code>. This is because the comma is within parentheses.
      </p>
<p>
        In the case of using the macro to introspect DClassTemplate the number of
        Boost PP array parameters is 2, because there is a comma separating the tokens
        in <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">T</span></code>.
      </p>
<h5>
<a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.h3"></a>
        <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.variadic_macro_usage"></a></span><a class="link" href="tti_detail_has_template.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.variadic_macro_usage">Variadic
        macro usage</a>
      </h5>
<p>
        Having the ability to use variadic macros makes the syntax for using BOOST_TTI_TEMPLATE
        easier to specify in both the <code class="computeroutput"><span class="keyword">template</span>
        <span class="identifier">type</span> <span class="identifier">parameters</span></code>
        form and the <code class="computeroutput"><span class="identifier">specific</span> <span class="identifier">parameters</span></code>
        form of using the macro. This is because variadic macros can take a variable
        number of parameters. When using the variadic macro form the first macro
        parameter is always the name of the class template you are trying to introspect.
        You only specify further parameters when using the <code class="computeroutput"><span class="identifier">specific</span>
        <span class="identifier">parameters</span></code> form of the macro, in
        which case the further parameters to the macro are the specific template
        parameters.
      </p>
<p>
        Introspecting the first class template above using the <code class="computeroutput"><span class="keyword">template</span>
        <span class="identifier">type</span> <span class="identifier">parameters</span></code>
        form the variadic macro would be:
      </p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">AClassTemplate</span><span class="special">)</span>
</pre>
<p>
        Introspecting the other class templates above using the <code class="computeroutput"><span class="identifier">specific</span>
        <span class="identifier">parameters</span></code> form the variadic macros
        would be:
      </p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">BClassTemplate</span><span class="special">,</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">class</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)</span>
<span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">CClassTemplate</span><span class="special">,</span><span class="keyword">void</span> <span class="special">(*)(</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">long</span><span class="special">))</span>
<span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">DClassTemplate</span><span class="special">,</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">class</span><span class="special">)</span>
</pre>
<p>
        Here we have no problem with counting the number of tuple tokens for the
        Boost PP array, nor do we have to specify BOOST_PP_NIL if we are using the
        <code class="computeroutput"><span class="keyword">template</span> <span class="identifier">type</span>
        <span class="identifier">parameters</span></code> form. Also for the specific
        parameters form we simply use the template parameters as the remaining tokens
        of the variadic macro.
      </p>
<h5>
<a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.h4"></a>
        <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.the_resulting_metafunction"></a></span><a class="link" href="tti_detail_has_template.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_macro.the_resulting_metafunction">The
        resulting metafunction</a>
      </h5>
<p>
        Using either form of the macro, whether using variadic or non-variadic syntax,
        the macro generates a metafunction called "has_template_'name_of_inner_class_template'".
      </p>
<p>
        The metafunction can be invoked by passing it the enclosing type to introspect.
      </p>
<p>
        The metafunction returns a single type called 'type', which is a boost::mpl::bool_.
        As a convenience the metafunction returns the value of this type directly
        as a compile time bool constant called 'value'. This is true or false depending
        on whether the inner class template exists or not.
      </p>
</div>
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<td align="right"><div class="copyright-footer">Copyright © 2011-2013 Tropic Software
      East Inc<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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